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Normal Distribution Calculations

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State: Express the problem in terms of the observed variable x. Plan: Draw a picture of the distribution and shade the area of interest under the curve. Do: Perform calculations. Standardize x to restate the problem in terms of a standard Normal variable z. Use Table A and the fact that the total area under the curve is 1 to find the required area under the standard Normal curve. Conclude: Write your conclusion in the context of the problem. How to Solve Problems Involving Normal Distributions

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When Tiger Woods hits his driver, the distance the ball travels is normally distributed with mean distance of 304 yards and standard deviation of 8 yards. What percent of Tiger’s drives travel between 305 and 325 yards? Using Table A, we can find the area to the left of z = 2.63 and the area to the left of z = 0.13. 0.9957 – 0.5517 = 0.4440. About 44% of Tiger’s drives travel between 305 and 325 yards.

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Example 2 4 A Company produces “20 ounce” jars of picante sauce. The true amounts of sauce in the jars of this brand follow a normal distribution. Suppose the company’s “20 ounce” jars follow an N (20.2, 0.125) distribution curve.

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Example 2: N (20.2, 0.125) a. What proportion of the jars are under-filled (i.e., have less than 20 ounces of sauce)? 5 Now write the probability statement: Show calculator syntax: First, standardize the value of 20 by finding its z-score:

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Example 2: N (20.2, 0.125) 6 b. What proportion of the sauce jars contain between 20 and 20.3 ounces of sauce? Write the probability statement and show syntax: Find z-scores for both values:

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Example 2: N (20.2, 0.125) 7 c. 99% of the jars of this brand of picante sauce will contain more than what amount of sauce? We first use InvNorm to find the corresponding z-score: Now we use the z-score formula to find x:

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Example 3 The cars in Clunkerville have a mean age of 12 years and a standard deviation of 8 years. What percentage of cars are more than 4 years old?

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Example 4 SAT I math scores are scaled so that they are approximately normal, with mean about 511 and standard deviation about 112. A college wants to send letters to students scoring in the top 20% on the exam. What SAT I math score should the college use as the dividing line between those who get letters and those who don’t?

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Normal Distributions. Homework Answers 1. a) Her percentile is.25, or the 25 th percentile. 25% of girls have fewer pairs of shoes b) His.

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